A Systematic Review on Traditional Uses, Sources, Phytochemistry, Pharmacology, Pharmacokinetics, and Toxicity of Fritillariae Cirrhosae Bulbus

Fritillariae Cirrhosae Bulbus (known as chuanbeimu in Chinese, FCB) is a famous folk medicine which has been widely used to relieve cough and eliminate phlegm for thousands of years in China. The medicine originates from dried bulbs of six species of Fritillaria which are distributed in the temperate zone of the Northern Hemisphere. Increasing attention has been paid to FCB because of its excellent medicinal value such as being antitussive, expectorant, analgesic, anticancer, anti-inflammatory, and antioxidative. During the past years, a large number of research studies have been conducted to investigate the phytochemistry, pharmacology, and pharmacokinetics of FCB. A range of compounds have been isolated and identified from FCB, including alkaloids, saponins, nucleosides, organic acids, terpenoids, and sterols. Among them, alkaloids as the main active ingredient have been illustrated to exert significant therapeutic effects on many diseases such as cancer, acute lung injury, chronic obstructive pulmonary disease, asthma, Parkinson's disease, and diabetes. Due to the excellent medical value and low toxicity, FCB has a huge market all over the world and triggers a growing enthusiasm among researchers. However, there is still a lack of systematic review. Hence, in this work, we reviewed the FCB-based articles published in Sci Finder, Web of Science, PubMed, Google Scholar, CNKI, and other databases in the recent years. The traditional uses, sources, phytochemistry, pharmacology, pharmacokinetics, and toxicity of FCB were discussed in the review, which aims to provide a reference for further development and utilization of FCB.


Introduction
Fritillariae Cirrhosae Bulbus (FCB) is one of the best known Chinese herbal medicines used to treat respiratory disease and obtained from the Fritillaria species [1][2][3] [4]. e six Fritillaria plants are characterized by poor growth and strict environmental requirements. Also, they are mainly distributed between the altitudes of 3000 and 5000 m in most parts of Tibet, Northwest Sichuan, Northern Yunnan, and Southeastern Qinghai [5,6].
Historically, FCB was firstly documented in "Shen Nong Ben Cao Jing" (a classic medical book of the Han dynasty) and has been widely used clinically owing to its remarkable antitussive, expectorant, and antiasthmatic activities [7]. So far, over 200 products containing FCB in the market have been applied for the treatment and prevention of a variety of diseases such as cough and asthma [8]. As the development of separation and identification technique, numerous phytochemical investigations concerning FCB have been carried out during the past several decades. Also, many constituents derived from FCB have been identified, including steroidal alkaloids, saponins, terpenoids, and glycosides [9]. Among

Sources
According to the Chinese Pharmacopoeia (2015), FCB is derived from the dried bulbs of six Fritillaria species, including FC, FP, FU, FD, FT, and FW [4] (Figure 1). It is worth noting that these Fritillaria species are generally perennial herbs and have strict requirements for the growing environment. Among them, FC commonly grows in high altitude habitats from 3200-4200 m, has a disjunct distribution across the high mountain areas of Southern and Eastern Tibet, northwestern Yunnan, and western Sichuan. FP is mainly distributed at altitudes between 2800 and 4400 m in Southern Gansu, eastern and southern Qinghai, and western Sichuan. FU usually grows at the altitudes between 3200 and 4500 m in northwest of Sichuan and southeastern of Qinghai. FD is chiefly distributed at the altitudes between 3800 and 4700 m in northwestern Yunnan, western Sichuan, southern Qinghai, and Tibet. FT primarily grows at the altitudes between 2400 and 3150 m in Shaanxi (Qinling Mountains and its south), southeastern Gansu, northeastern Sichuan, and northwestern Hubei [24]. FW is mainly distributed at altitudes between 2500 and 3600 m in northwestern Sichuan [25] (Figure 2). Based on the difference in botanical origins and producing areas, FCB is divided into different types (including songbei, qingbei, and lubei) which have different efficacies and prices. It has long been acknowledged that songbei has the highest medicinal value and excellent quality [26,27]. In the recent years, the enhancement of health consciousness has induced an increase of the demand of FCB. Also, over 2000 tonnes of FCB was used in China each year [8]. e production of medicinal preparations containing FCB was an industry with an estimated value of US $400 million per year [28]. However, the sources of wild FCB is inadequate for the increasing domestic and international market demand because the original plants of wild FCB grow extremely slow, and they usually require, at least, five years to grow into an apparent size [21]. As a result, extensive commercial use and limited wild stocks lead to a high price of FCB. From 2013 to 2016, the price of FCB has reached 3000 Yuan (RMB) per kg [14]. Terribly, the huge market demand and high profit of FCB further exacerbated the overexploitation of wild sources. Nowadays, these used species in Fritillaria genus have been classified as precious, rare, and threatened species by the Endangered Species Scientific Commission of China in 2012 [28].

Phytochemistry
e earliest research concerning the chemical composition of Fritillaria species could date back to 1888. In this year, imperialine, an alkaloid, was firstly isolated from F. Shedan Chuanbei San Fritillariae Cirrhosae Bulbus and snake bile Clearing lung, relieving cough, and dissolving phlegm [4] Evidence-Based Complementary and Alternative Medicine imperialis [29]. Subsequently, extensive chemical studies have been conducted on various Fritillaria species, especially on the TCM herbal chuaneimu. To date, more than 100 chemical components (mainly including alkaloids, saponins, nucleosides, organic acids, terpenoids, and sterols) have been extracted and identified from FCB (shown in Table 2).

Alkaloids.
Alkaloids are a group of naturally occurring chemical compounds that contain cyclic structures with, at least, one basic nitrogen atom being incorporated within, which have a wide distribution in medicinal plants [39]. As the fundamental components of FCB, alkaloids are traditionally believed to be responsible for the pharmacological activities of FCB [34] and have been considered as a key index for the identification and quality control of FCB [36]. Interestingly, the content of total alkaloids in FCB is extremely low, approximately ranging from 0.02% to 0.3% [40]. Previous studies showed that the alkaloids from FCB could be classified into two groups based on the carbon framework: isosteroidal alkaloids and steroidal alkaloids; the former accounts for approximately 75% of the total alkaloids [18,41]. In the last several decades, many phytochemical research studies have been carried out to extract and identify the alkaloids in FCB. To date, more than 30 alkaloids in FCB have been identified, mainly including imperialine, peimisine, peiminine, peimine, and chuanbeinone. Figure 3 shows the details of that.

4.2.
Saponins. e saponins are natural surface-active glycosides existing in many popular medicinal plants, which have been regarded as a kind of promising chemical component because of its various pharmacological properties, such as immunomodulatory, antioxidative, antidiabetic, and anti-cancer properties [42]. In the recent years, some studies reported that saponins may be one of the active ingredients responsible for the pharmacological activities of FCB. e content of total saponins in FCB is approximately ranging from 3% to 4% [43]. However, a few research studies focused on the recognition of saponins.

Nucleosides.
Nucleosides belong to a class of organic compounds with a nitrogen-containing heterocyclic nucleobase and a 5-carbon sugar [44]. ey have attracted Astringing lung, dissolving phlegm, and relieving cough and asthma [4] attention because of their multiple pharmacological effects, such as antitumor, antithrombotic, anti-inflammatory, and anti-fungal effects [45]. Recent chemical investigations suggested that nucleosides are present in the water extract of Fritillaria Bulbs [36], which indicates that nucleosides may participant in the pharmacological activities of FCB. To date, more than 10 nucleosides have been isolated and identified from FCB, such as uracil, thymine, cytidine, inosine, and thymidine. e details of that are shown in Figure 4.

Organic Acids.
Organic acids are also a kind of important compounds existing in FCB. At present, a variety of organic acids have been derived from FCB, and the chemical structure is shown in Figure 5.

Sterols and Terpenoids.
In addition to alkaloids, saponins, nucleosides, and organic acids, sterols and terpenoids also have been found in many Fritillaria species ( Figure 6).

Pharmacology
Recently, FCB is attracting more and more attention because of its excellent medicinal value and extremely low toxicity. Based on the evidence of previous in vivo or in vitro studies, FCB exerts a series of biological activities, including antitussive, expectorant, analgesic, anticancer, anti-inflammatory, and antioxidative activities, which is summarized in Figure 7 and Table 3.

Anticancer.
Cancer is a formidable disease with high morbidity and mortality [75]. Currently, there are several remedies, mainly including chemotherapy, surgery, and radiation therapy. However, severe adverse effects caused by these remedies are unavoidable [76]. erefore, it is an extreme active domain to search for the anticancer agents with higher bioactivity and lower toxicity in the future. Interestingly, concern pharmacologic studies have indicated that FCB is a potential candidate in the treatment of several types of cancers, such as lung cancer, colorectal cancer, liver cancer, endometrial cancer, oral cancer, ovarian cancer, and myelogenous leukemia.

Lung Cancer.
Currently, lung cancer as the most common cancer is attracting increasing attention [77]. In previous studies, FCB or its active compounds have shown significant inhibitory effects on the development of lung cancer. One in vivo study suggested that the total alkaloids of FCB (TFCB) (10-40 mg/kg/day for10 days) markedly suppressed the growth of transplantable Lewis lung carcinoma (LLC) tumor in mice through inhibiting tumor angiogenesis and inducing apoptosis. e molecular mechanism was related to the downregulation of endothelial cell adhesion molecule-1 (CD31) and caspase-3. Furthermore, the antitumor effects of TFCB were also confirmed by an in vitro study. It was observed that TFCB (30 μg/mL for 0-72 h) exhibited inhibitory effects on the proliferation activities of LLC cells, which may partly result from the apoptosis induction mediated by S-phase cell cycle arrest [47]. Besides, accumulated evidence pointed out that the aqueous extract of FCB (FC-AE) also has antitumor effects similar to those of TFCB. It was demonstrated that FC-AE (0.2 mL/2.5 mg/mL/ 2 day for 20 days) performed suppression activities on the growth of transplantable A549 tumors in a nude mice model, which was associated with the enhancement of signal transducer and activator of transcription (STAT) 1, STAT4, interferon gamma (IFNc), interleukin-(IL-) 12, caspase-3, B-cell lymphoma-2-associated X apoptosis regulator (Bax) levels, and the decrease of B-cell lymphoma-2 (Bcl-2) production. Moreover, FC-AE (0-100 μg/mL for 48 h) exhibited excellent antiproliferative effects on A549 cells, which was attributable to the accumulation of G2 phase cells and the promotion of Bax, STAT1, and STAT4 activities, as well as the downregulation of Bcl-2 proteins [48]. In addition to total extracts, some monomer chemical compositions, such as chuanbeinone and imperialine, also possess huge therapeutic potential on the treatment of lung carcinoma. For example, chuanbeinone (5-15 μg/mL for 48 h) dramatically reduced the viability of LLC cells, and the positive results were accompanied with an accumulation of cells in the S phase and a decrease of the cells in the G0/G1 phase, which subsequently leads to the augmentation of apoptosis. e underlying mechanism was related to the reduction of antiapoptotic protein Bcl-2 and the enhancement of proapoptotic protein Bax and caspase-3. To better understand the role of chuanbeinone on the treatment of lung cancer, an in vivo study suggested that chuanbeinone (10-40 mg/kg/day for 10 days) significantly suppressed the growth of transplantable LLC tumors in mice. e antagonistic effects were attributed to inactivation of CD31-mediated tumor angiogenesis, as well as the promotion of caspase-3-mediated apoptosis [49]. Similarly, extensive studies have demonstrated the antitumor effects of imperialine. A recent study showed that imperialine (200 ng/mLfor 24 h) notably inhibited the proliferation of human lung adenocarcinoma cell line A549 via downregulating the key regulatory molecules in the nuclear factor-κB (NF-κB) pathway, including phosphoinositide 3kinase (PI3K) Class III, protein kinase B (Akt), p-Akt, nuclear factor-κB-inducing kinase (NIK), IκB kinase (IKK) α&β, nuclear factor κBα (IκBα), and decreasing the expression of Ki67 (a clinical biomarker for tumor progress evaluation), as well as upregulating the levels of caspase-3. Furthermore, imperialine (10 mg/kg for 18 days) prominently blocked the development of non-small-cell lung cancer in the mice model, which involves the downregulation of the levels of inflammatory cytokines, such as IL-1β, IL-6, tumor necrosis factor-α (TNF-α), and the decrease of Ki67 [50].

Colorectal Cancer.
As a major bioactive constituent of FCB, peiminine has been validated to exert promising antitumor effects. Based on the past studies, peiminine (3 mg/ kg/2 days for 14 days) could significantly repress the growth of transplantable HCT-116 tumor in mice through inducing apoptosis and autophagy. e molecular mechanism was related to the enhancement of microtubule-associated protein light chain3B (LC3B) and cleaved caspase-3 levels. Analogously, it was discovered that the cell viability was dose-dependently decreased in colorectal carcinoma HCT-116 cells treated with peiminine (0-400 μM for 24 h). e positive results may ascribe the elevation of the ratio of LC3B-II/LC3B-I and the upregulation of phosphorylated Unc51-like kinase 1 (p-ULK1), phosphorylated adenosine 5′-monophosphate-activated protein kinase (p-AMPK), caspase-9, and cleaved caspase-3 levels, as well as the damping of phosphorylated mammalian target of rapamycin (p-mTOR), p-Akt, phosphorylated phosphatase, and tensin homolog (p-PTEN) activities [51]. Furthermore, peiminine (0-400 μM for 48 h) could trigger the death of HCT-116 cells by promoting apoptosis and autophagic flux via modulating the production of metabolites, such as glucose, glutamine, oleate, and lignocerate [52].

Liver
Cancer. An accumulating line of evidence indicates that FCB is a potential antitumor agent. In previous studies, various fractions from FCB, mainly including   Evidence-Based Complementary and Alternative Medicine   [54]. Additionally, another study denoted that FC-AE (200 mg/mL for 96 h) prominently inhibited the viability of ovarian and endometrial cancer cells, which involves the accumulation of S-phase cells and the increase of apoptotic cells. Further investigation showed that FC-AE attenuated the expression of cyclin D1, cyclin D3, NF-κB subunit p50 (NF-κBp50), p-IκBa, cysteine-X-cysteine chemokine receptor 4 (CXCR4), and matrix metalloproteinase-9 (MMP-9) and augmented the expression of p27 and caspase-3. e results given above indicate that FC-AE suppresses progression of endometrial cancer and ovarian cancer by activating apoptotic pathways and cell cycle arrest, as well as abrogating the NF-κB pathways activation [55]. Of course, some monomer chemical compositions have also been reported to exhibit advantageous effects on the treatment of endometrial cancer or ovarian cancer. A recent research suggested that peimisine (15 μg/mL for 0-72 h) notably blocked the proliferation of human ovarian cancer cell through inducing apoptosis via increasing G0/G1 phase cell arrest [56].

Other Cancers.
Noteworthy, FCB also displays advantageous effects on the treatment of other types of cancers, such as oral cancer and myelogenous leukemia. Based on the published literatures, peimine (50 μg/mL for 3 days) could suppress the proliferation of immortalized and malignant oral keratinocytes through inducing cell cycle arrest and apoptosis. e molecular mechanism was connected with the downregulation of Bcl-2 and retinoblastoma protein (pRb) and the upregulattion of Bax, caspase-3, p53, and p21 protein [57]. Furthermore, Pae et al. has reported the inhibitory effects of peimine on the growth of Human Promyelocytic Leukemia HL-60 Cells [58]. According to the in vitro or in vivo studies mentioned above, FBC or its extracts or its active components exhibit antitumor effects by inducing apoptosis and cell cycle arrest, as well as enhancing autophagic flux. e anticancer mechanism of FCB is shown in Figure 8.

Respiratory Disease.
Respiratory diseases are one of the most important causes of morbidity and mortality worldwide [79]. FCB has been wildly used as folk medicine in China for a long time due to its significant therapeutic effects on various respiratory diseases, such as cough, expectoration, pneumonia, bronchial inflammation, and asthma.

Cough.
Cough is a crucial defense mechanism which maintains airway patency and eliminates the potential harmful stimuli from the airway and lung [80]. Generally, cough is mostly caused by the common cold [81]. FCB has been used to suppress cough in China for thousands of years. A large number of studies have been carried out to explore the ameliorative effects of FCB on cough. An in vivo study has evaluated the antitussive activities of the alkaloids derived from FCB. Also, the results indicated that imperialine, chuanbeinone, peimine, and peiminine (1.5, 3.0 mg/kg for 1 h) obviously enhanced the latent period of cough and inhibited the cough frequency in mice [59]. Similarly, another research has confirmed that imperialineβ-N-oxide, isoverticine, and isoverticine-β-N-oxide (1.5-4.5 mg/kg for 1 h) establishe mitigative effects in mice with cough [60]. Additionally, a recent study suggested that the crude alkaloid and FC-AE performed a dose-dependent tracheobronchial relaxation on the carbachol precontracted rat-isolated tracheal and bronchial rings [61]. Moreover, according to the findings of computational target fishing, mitogen-activated protein kinase 1 (MAPK1), Akt1, and PPKCB may be important targets of peimine for the treatment of cough [82].    Figure 3: e chemical structure of alkaloids from FCB.   Evidence-Based Complementary and Alternative Medicine 13   Analogously, Wang et al. illustrated that imperialine-β-Noxide, isoverticine, and isoverticine-β-N-oxide (1.5-4.5 mg/ kg for 0.5 h) significantly promoted tracheal phenol red output, which indicated that the expectorant effects of FCB may be related to its ability to increase tracheobronchial mucus secretion [60]. e abovementioned results are consistent with the traditional use of FCB as a remedy for expectoration.

Acute Lung Injury.
As the research continues, some studies have reported that FCB is a promising agent to treat acute lung injury. TFCB (15-60 mg/kg/day for 5 days) could markedly attenuate LPS-induced acute lung injury in mice, which was associated with a decrease of TNF-α and IL-6 [62]. Furthermore, peiminine (0.005 g/kg for 28 days) has been demonstrated to exert inhibitory effects on the bleomycin-induced lung injury rat model by reducing the levels of IFN-c, TGF-β, connective tissue growth factor (CTGF), extracellular signal-regulated kinase (ERK)1/2, NF-κB, and Fas Ligand (FasL) [63]. Consistent to the results, another study found that peiminine (0-5 mg/kg for 6 h) attenuated histopathological changes and inhibited the expression of TNF-α, IL-1β, IL-6, and IL-8 in the LPS-induced acute lung injury mice model. Also, those changes may partly be relevant for the weakening of AKT/PI3K phosphorylation and the attenuation of lipid rafts formation [64].

Chronic Obstructive Pulmonary Disease (COPD).
COPD has long been considered as a severe public health problem. It is urgent to find new compounds for minimizing the risk of this disease [83]. Fortunately, it was shown that imperialine (3.5, 7.0 mg/kg/twice/day for 60 days) could mitigate pulmonary functional and structural impairment in lung tissues of the COPD-like rats. Also, the protective effects were result from the reduction of IL-1β, IL-6, IL-8, TNF-α, NF-κB p65, TGF-1β, MMP-9, and tissue inhibitor of metalloproteinase-1 (TIMP-1) [65]. Besides, peiminine has also been substantiated to prevent the exacerbation of COPD by decreasing the expression of p-AKT and phosphorylated glycogen synthase kinase 3β (p-GSK3β) while increasing the expression of phosphorylated myosin light chain2 (p-MLC2). Increasingly, an FCB-based traditional Chinese medicine formula, Chuan Bei Pi Pa dropping pills (50-200 mg/kg for 35 days), has been demonstrated to block the progression of COPD through decreasing the number of leukocytes [66].

Asthma.
Asthma is a chronic inflammatory disorder of the airways which has been regarded as a formidable challenge for clinicians. At present, the therapeutic effects of conventional medicine are still unsatisfactory [84]. Noteworthily, an in vivo study has indicated that an aqueous extract of FCB (200 mg/kg/three times/week for 56 days) could inhibit the development of asthma in mice by suppressing 2 cytokines (IL-4, IL-5, and IL-13), IgE, histamine production, and reducing eosinophilic accumulation, as well as increasing of interferon-c production [67] ( Figure 9).

Pain.
In the previous studies, some research has been conducted to evaluate the analgesic effects of isosteroidal alkaloid from FCB. Peimine as an important ingredient of FCB has been confirmed to exert significant analgesic effects on acetic acid-/formalin-/paclitaxel-induced nociception pain in mice, which indicates that peimine may be a plausible candidate to relieve inflammatory pain and cancerrelated neuropathic pain [85].
Evidence-Based Complementary and Alternative Medicine 5.6. Other Pharmacological Effects. FCB is a natural agent with various pharmacological effects; in addition to the abovementioned pharmacological effects, other biological activities have also been reported in many researches in the last few decades. For instance, peiminine (0-5 mg/kg for 28 days) markedly attenuated behavioural dysfunction and inhibited the loss of dopaminergic neurons and microglial activation in the LPS-induced Parkinson's disease rat model. Moreover, it was ascertained that peiminine (0-50 μg/mLfor 13 h) obviously decreased the expression of the proinflammatory mediators TNF-α, IL-6 and IL-1β, COX-2, and iNOS by inhibiting the phosphorylation of ERK1/2, AKT, and NF-κB p65 in LPS-induced BV-2 cells [72]. Additionally, peiminine (0-5 mg/kg for 13 h) could exert attenuation effects on LPS-induced mastitis in mice. e positive results were related to downregulation of myeloperoxidase, TNF-α, IL-6, IL-1β, COX-2, iNOS, AKT, NF-κBp65, ERK1/2, and p38. Consistent to the results, peiminine (0-70 μg/mL for 4 h) inhibited the secretion of proinflammatory cytokines in LPS-induced mouse mammary epithelial cells via repressing AKT/NF-κB, ERK1/2, and p38 signaling pathways [73]. Based on the result of a recent study, peimine (0-100 μg/mL for 24 h) performed hypoglycemic activities by enhancing the levels of insulin and glucose uptake ability in b-TC6 pancreatic and C2C12 skeletal muscle cells, which implies that peimine has huge therapeutic potential in diabetes [74].

Pharmacokinetics of FCB
In the past few decades, the pharmacokinetics studies of FCB mainly focused on its alkaloids, such as imperialine, peimisine, and peimine. Imperialine has high oral bioavailability, and a previous study have shown that the bioavailability of administered imperialine at an oral dose of 1 mg/kg, 5 mg/kg, and 10 mg/kg was 31.2%, 53.6%, and 47.4%, respectively [86]. However, the half life of imperialine is short, and if prepared, imperialine is supposed to be taken, at least, at three times a day, which will certainly degrade the patient compliance. Nevertheless, the sustained-release imperialine tablets based on hydroxypropylmethylcellulose could significantly prolong the drug action time and simultaneously improve the oral bioavailability [87]. To further evaluate the intestinal absorption characteristics of imperialine, an in vitro study found that the uptake of imperialine was increased with increasing PH and concentration in Caco-2 cells. Also, some transporters, such as P-glycoprotein and Niemann-Pick C1-Like 1, did not participate in the absorption. Additionally, based on the in situ assessments, the absorption of imperialine varied in four intestinal segments (duodenum, jejunum, ileum, and colon). Among them, the colon displayed the highest level of absorption [88]. Automatically, the pharmacokinetics of peimisine in rats has been assessed by liquid chromatography-tandem mass spectrometry. Also, the results suggested that peimisine was slowly distributed and eliminated from rat plasma and showed linear dynamics in a dose range of 0.26-6.5 mg/kg. Moreover, peimisine was primarily distributed in the spleen, liver, kidney, heart, and lung. Intriguingly, the drug blood and tissue levels in male rats were apparently higher than the female counterparts after oral administration. Moreover, it was confirmed that the major elimination route of male rats was urine excretion after oral peimisine [89]. Similarly, Wu et al. have evaluated the pharmacokinetics, tissue distribution, and excretion of peimine in rats. e results showed that the oral bioavailability in male rats (45.8%) was much higher than in female rats (2.74%). Nevertheless, peimine was metabolized more extensively in female rats than in male rats, and peimine had good tissue penetrability and high tissue affinity in most studied tissues [90]. Furthermore, Chen et al. attempted to investigate the intestinal absorption of peimine in Caco-2 cells. ey found that peimine transport was concentration dependent and was increased with increasing PH (6.0-7.4), and P-glycoprotein may participant in the transport of peimine [91]. Some pharmacokinetic parameters of imperialine, peimisine, and peimine are summarized in Table 4, including maximum concentration (C max ), elimination halflife (T1/2), time to reach C max (T max ), oral clearance (CL/F), volume distribution/bioavailability (V/F), and area under the concentration-time curve (AUC) from zero to t and from zero to infinite time.

Toxicity of FCB
Over the past several millennia, it is generally assumed that FCB is a nontoxic or low toxic herb. To date, few studies focused on the toxicity of FCB. Recently, the acute oral toxicity of FCB has been assessed by an in vivo study. e results revealed that the maximum feasible dose value of FCB in mice was 452.14 g/kg. ere were no mice dead after administering the FCB extract at the maximum feasible dose for 14 days. In the histological examination, only the liver showed significant pathological changes [92]. Additionally, Guo et al. have evaluated the toxicity of FCB on human NCM460 colon epithelial cells. It was demonstrated that FC-AE (80 and 160 mg/mL for 72 h) caused mitotic aberrations and genomic instability by inducing spindle assembly checkpoint dysfunction [93]. As the research moves along, it was found that FC-AE (80 and 160 μg/mL for 72 h) leaded to cytokinesis failure through triggering chromosomal instability via promoting the incidence of binucleated cells in NCM460 cells [94]. In summary, the use of FCB should be adopted by appropriate dosage and course of treatment.

Future Perspectives and Conclusions
FCB is a famous TCM with a wide spectrum of pharmacological effects and has been extensively used as a major ingredient in cough syrups with a huge commercial market. At present, FCB is still mainly harvested from the wild fields. However, the wild Fritillaria species used as the botanical origins of FCB grow extremely slow. Also, they are difficult to meet the increasing market demands [95]. In addition, the overexploitation of wild resources results in the extinction of many Fritillaria species. erefore, it is an urgent problem to find out an alternative way to improve the production of FCB. Encouragingly, the emerging of artificial cultivation technique has partly solved the shortage of supplies. Also, the content of total alkaloids in cultivated FCB is higher than that in wild FCB [29]. But, many shortcomings remain; for example, large-scale cultivation of those species is limited because of the restricted growth conditions. Additionally, the output of artificial cultivation is not ideal whereas the cost of that is high, and it still requires a long time to grow into an apparent size [96]. Moreover, the practical value of Fritillaria propagation by seed is low because the growth of seedlings in natural habitats is too weak [97]. Recently, tissue culture has been considered as a promising approach because it dramatically reduces the breeding time of Fritillaria species. But, there is a fly in the ointment: the stages of rooting and transplanting are difficult, which may result from dormancy of bulbs [98]. Hence, the propagation of Fritillaria is another point worth taking into consideration. Of course, searching for the substitutes is also a feasible way  In the traditional viewpoint, it is commonly acknowledged that the major bioactive constituents of FCB are alkaloids. Also, a large proportion of pharmacological and chemical studies are focused on them. Moreover, the content of alkaloids has been regarded as an important index to evaluate the qualities of FCB. However, the chemical researches have shown that FCB has the lowest levels of alkaloids compared with other Fritillaria species [43]. erefore, it is a pressing issue to verify whether alkaloids are the most representative of the characteristic pharmaceutical components of FCB. Interestingly, the content of saponins in FCB is higher than any Fritillaria species [43], which indicates that saponins may play a pivotal role in the pharmacologic activities of FCB. Hence, it is necessary to conduct more pharmacologic studies to investigate the biological activity of saponins and other compounds from FCB, and more chemical investigations are recommended to concentrate on the other compounds of FCB.
In the past few decades, extensive studies have attempted to assess the pharmacological effects of FCB. To our best knowledge, FCB exerts unexceptionable therapeutic effects on various diseases, including cough, expectoration, acute lung injury, chronic obstructive pulmonary disease, asthma, and some type of cancers, such as lung cancer, colorectal cancer, liver cancer, endometrial cancer, and ovarian cancer. Nevertheless, the underlying mechanisms of its pharmacology actions are still unclear. For instance, the antitussive and expectorant activities have been demonstrated in the previous studies, but the hidden mechanism is not fully understood. erefore, we should conduct more molecular studies to gain a broader knowledge on the mechanism of FCB's pharmacologic action in future. Additionally, there are not enough studies regarding the pharmacokinetics and toxicity of FCB or its active ingredients. More profound evaluations are required to investigate the pharmacokinetics of other constituents besides alkaloids from FCB. Also, the toxicity studies focusing on molecular levels are still a promising direction.
In the recent years, the growing market demand leads to a rapid rise of the price of FCB. e price of FCB was beyond $260 per kilogram in 2014, almost nine-folds of that in 2004, and it is much higher than other Fritillariae Bulbus [28]. In order to obtain more profits, some merchants adulterate other Fritillariae Bulbus to sell as FCB on commercial markets. However, it is very difficult to distinguish those fakes from FCB by conventional morphological analyses because they have similar morphological characteristics [100,101]. Recently, some identification methods based on chemical analyses, such as high-performance liquid chromatography (HPLC), have been used to identify and determine isosteroidal alkaloids, which are also inadequate for distinguishing Fritillariae species because the content of chemical compositions is often affected by different practices of planting, harvesting, storage, and manufacturing [102,103].
us, an accurate and efficient identification method should be imminently explored to control the quality of this herb and ensure the drug safety of FCB. Currently, DNA barcodes have been confirmed to be an effective tool for rapid and accurate species discrimination. It could identify plants species by employing a short and standardized DNA region [104,105]. Based on the previous investigation, ITS2 sequences (a DNA barcode) could    Figure 10: e anti-inflammation mechanism of FCB ("↑"represents the increase, "↓" represents the decrease). successfully distinguish the six original species of FCB from other Fritillariae species. Nevertheless, it is incapable to distinguish the six original species of FCB from each other [106]. erefore, it is necessary to establish a method for comprehensive differentiation and evaluation of FCB from different varieties in terms of physical properties, chemical compositions, molecular markers, and pharmacological activities.
To sum up, this work is the first to systematically review the traditional uses, sources, phytochemistry, pharmacology, pharmacokinetics, and toxicity of FCB. e recent findings about FCB have been summarized in this work, to provide a reference for further development and utilization of FCB.

Conflicts of Interest
e authors declare no conflicts of interest.